You can actually do it in ABC - not sure how portable it is, but BarFly lets you specify any of the MIDI sounds as the timbre to use for a particular voice. Play the Stool of Repentance entirely in slaps of various pitches, make your own doggie Christmas carols, Listen to the Mockingbird in tweets, or sing What a Friend We have in Jesus in a chorus of phone rings, hoofbeats, door creaks and manic laughs.